Over 4th of July we drove down over the Sagamore and it was fine -- no backups. Coming back, we went across the Bourne bridge.
We stayed for 4 days in New Seabury outside of Mashpee.
Here's a link to some traffic cameras we kept an eye on:
http://www.capecodlivecam.com/index.shtml
Enjoy your trip. I highly recommend the whale watching tour out of Barnstable!